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1.
The kinetics of thermal instability in the form of an initially arising local region of normal conductivity is investigated
using a model of a discrete superconducting medium in which the components are assumed to interact thermally across a thermally
thin intermediate layer. It is found that certain characteristic features of the transient processes occurring at currents
close to the minimum current for propagation of the normal zone must be taken into account in an experimental determination
of the normal-zone propagation velocities in multiconductor composites.
Zh. Tekh. Fiz. 69, 133–136 (February 1999) 相似文献
2.
V. R. Romanovskii 《Technical Physics》1998,43(1):47-51
The conditions for the irreversible propagation of a normal zone along a composite superconductor are investigated within
the model of a continuous medium with consideration of its transverse thermal conductivity under the assumption of a uniform
distribution of the current over the cross section of the wire. The numerical experiments performed for a currentcarrying
element of circular cross section with variation of the cooling rate and the transverse dimensions are compared with known
results of the one-dimensional theory. It is shown that the one-dimensional theory, as opposed to the two-dimensional theory,
leads to underestimated values of the velocity of a thermal instability. The size effect modifies the propagation conditions
of a normal zone to the greatest extent as the heat-transfer coefficient increases. This law is based on an increase in temperature
at the center of the wire with a simultaneous decrease on its surface, as a result of which the mean temperature over the
cross section of the composite increases as its radius increases.
Zh. Tekh. Fiz. 68, 53–57 (January 1998) 相似文献
3.
An investigation was made of the influence of a nonconducting shell on the velocity of irreversible propagation of the normal
zone along a cooled composite superconductor. It is shown that even a coating of negligible thickness may lead to a substantial
deterioration in the thermal stabilization conditions of intensively cooled current-carrying elements.
Zh. Tekh. Fiz. 68, 95–97 (December 1998) 相似文献
4.
V. R. Romanovskii 《Technical Physics》1999,44(4):464-467
An investigation is made of the Joule dissipation of energy in the superconducting phase of a composite superconductor as
the current in it increases. It is shown that at high rates of current injection the dissipative processes are characterized
by an appreciable release of heat, which substantially influences the critical energies for the loss of superconductivity
and the velocity of the normal zone. An analytical expression is given for calculating the power of the thermal losses during
current injection.
Zh. Tekh. Fiz. 69, 125–128 (April 1999) 相似文献
5.
《Waves in Random and Complex Media》2013,23(2):257-282
Abstract We consider the statistics of the transverse spectra of forward-propagating waves in a stationary random medium. A short-range perturbation solution is used to derive the difference equations that govern the long-range evolution of the ensemble-averaged transverse wave spectrum and coherence. The conditions under which these equations may be approximated by differential and integro-differential equations are given, and it is shown that the approximation is valid for the treatment of beam propagation provided that the transverse dimension of the beam is sufficiently large, and at ranges where the transverse coherence length of the beam remains larger than a wavelength. The equations that are derived are not limited by the parabolic approximation, and are amenable to numerical solution by marching techniques. We use the equation that governs the spectral density of the total energy flux, and also the propagation of waves which are statistically homogeneous in transverse planes, to show the conditions under which previously studied approximations derive from the present formulation, and to illustrate the numerical solution of the problem. 相似文献
6.
V. R. Romanovskii 《Technical Physics》2017,62(11):1651-1659
The stability of dissipative states of a Bi2Sr2CaCu2O8 applied superconducting composite cooled by liquid helium or hydrogen with continuously increasing current-voltage characteristics described by a power equation has been studied. It has been shown that, under intensive cooling conditions, special conditions of thermal stabilization can exist for applied superconducting materials (technical superconductors) if the nonlinear temperature dependences of critical current density and specific resistance of the stabilizing matrix are taken into account. First, the minimum currents of existence and propagation of a normal zone can be absent. Second, the intensive cooling of a technical superconductor substantially increases the range of stable currents in the diapason of supercritical currents. Third, during the irreversible propagation of the thermal instability, the temperature of a technical superconductor can increase under conditions close to adiabatic, despite cooling by liquid coolant. These effects should be taken into account upon determining the conditions for overheating a technical superconductor. The numerical experiments have been compared with the results that follow from the thermal stabilization theory of combined superconductors, which assumes a linear temperature dependence of critical current density of a superconductor and a step-wise transition from the superconducting to normal state. 相似文献
7.
A. Stenvall R. Mikkonen P. Ková? 《Physica C: Superconductivity and its Applications》2010,470(22):2047-2050
Measured and simulated minimum quench energies (MQE) for short samples are doubtful when estimating disturbance tolerance of an impregnated superconducting magnet. If measurements are performed at liquid cryogen, the cooling provided by the cryogen causes MQE to be high. At a conduction-cooled system, the transverse heat conduction is neglected causing low MQE. Even though these conditions can be artificially brought closer to the ones in an impregnated magnet, it is difficult to estimate by short-sample measurement the MQE of a superconducting coil. These similar difficulties are present at simulations. On the other hand, 1D and 2D measurements and simulations can be used, e.g., to compare wires or consider effect of insulation layer thickness, but according to the results only in a rough quantitative way. In this paper, we compare MQE and normal zone propagation velocity simulations of an MgB2 wire with 1D, 2D and 3D finite element method models to focus on the different results given by the models employing different dimensions. 相似文献
8.
The propagation of two-component laser radiation in a medium consisting of atoms with a Λ level scheme is investigated. The
simultaneous self-focusing of two beams is considered. The main features of this phenomenon are: 1) lowering of the self-focusing
threshold by several orders of magnitude in comparison with the known case of saturation of a transition in a two-level atom;
2) a strong dependence of the character of the propagation of the radiation on the difference between the detunings of the
two frequency components of the field from resonance, which is associated with fulfillment of the two-photon resonance condition.
Zh. Tekh. Fiz. 67, 126–129 (July 1997) 相似文献
9.
Magnetogravitational instability of an infinite homogeneous, viscous, thermally conducting, rotating plasma flowing through a porous medium has been studied with the help of relevant linearized perturbation equations, using the method of normal mode analysis. Rotation is taken parallel and perpendicular to the magnetic field for both, the longitudinal and the transverse modes of propagation. The joint influence of the various parameters do not, essentially, change the Jeans' criterion but modifies the same. The adiabatic velocity of sound is being replaced by the isothermal one due to the thermal conductivity. Porosity reduces the effects of both, the magnetic field and the rotation, in the transverse mode of propagation, whereas the rotation is effective only along the magnetic field for an inviscid plasma. The viscosity removes the effect of rotation in the transverse mode of propagation. 相似文献
10.
V. V. Lysenko 《Technical Physics》2000,45(6):736-738
Influence of the current redistribution in the stabilizing matrix of a composite superconductor with nonuniform distribution of the superconducting component over its cross section on the normal zone propagation is calculated. Analysis of the problem in dimensionless variables allows one to find parameters determining the import of the effect considered. Parametric relationships for the normal zone propagation velocity are obtained under cooling conditions inadequate for steady state stabilization. 相似文献
11.
《Waves in Random and Complex Media》2013,23(4):437-450
We consider the intensity fluctuations arising when a point source of radiation moves in a randomly inhomogeneous scattering medium. The medium itself can also move with a velocity whose component normal to the direction of propagation can have an arbitrary distribution. We derive an expression for the space–time autocorrelation function of the intensity fluctuations transverse to the direction of propagation. The result is analysed for some particular cases and it is shown how the resulting information can be useful in examining the behaviour of random media in situations of practical interest. 相似文献
12.
O. é. Gulin 《Acoustical Physics》2010,56(5):613-622
A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is
presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value
problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal
coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium.
A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete
spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments,
which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated
propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model
with the use of adiabatic and one-way propagation approximations. 相似文献
13.
O.?é.?Gulin 《Acoustical Physics》2010,56(5):684-692
A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium. A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments, which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model with the use of adiabatic and one-way propagation approximations. 相似文献
14.
A. I. Belousov S. A. Berzakov Yu. E. Lozovik 《Journal of Experimental and Theoretical Physics》1998,87(2):322-328
The phase diagram of a two-dimensional Josephson array of mesoscopic objects (superconducting granules, superfluid helium
in a porous medium, traps with Bose-condensed atoms, etc.) is examined. Quantum fluctuations in both the modulus and phase
of the superconducting order parameter are taken into account within a lattice boson Hubbard model. Modulating the average
occupation number n
0 of the sites in the system (the “number of Cooper pairs” per granule, the number of atoms in a trap, etc.) leads to changes
in the state of the array, and the character of these changes depends significantly on the region of the phase diagram being
examined. In the region where there are large quantum fluctuations in the phase of the superconducting order parameter, variation
of the chemical potential causes oscillations with alternating superconducting (superfluid) and normal states of the array.
On the other hand, in the region where the bosons interact weakly, the properties of the system depend monotonically on n
0. Lowering the temperature and increasing the particle interaction force lead to a reduction in the width of the region of
variation in n
0 within which the system properties depend weakly on the average occupation number. The phase diagram of the array is obtained
by mapping this quantum system onto a classical two-dimensional XY model with a renormalized Josephson coupling constant and is consistent with our quantum path-integral Monte Carlo calculations.
Zh. éksp. Teor. Fiz. 114, 591–604 (August 1998) 相似文献
15.
为了及时的发现和保护超导体在失超过程中由于局部温度的突然升高而导致的材料的不可逆破坏行为,对于超导材料的失超行为深入地研究和定量地分析是必要的. 本文结合电场方程和热平衡方程, 建立了能够描述失超行为的热-电耦合方程, 并构建了能够表征多芯 Bi2212 圆线材结构的三维模型, 同时基于有限单元法进行数值计算, 研究其失超 行 为. 通过对比实验与研究结果验证模型的准确性和有效性, 继而研究了正常区传播速度(NZPV) 和最小失超能( MQE) 与运输电流之间的关系, 并给出了热失超过程中电流重分布过程. 相似文献
16.
N. N. Rozanov N. V. Vysotina A. G. Vladimirov 《Journal of Experimental and Theoretical Physics》2000,91(6):1130-1140
A variant of perturbation theory is developed to determine the characteristics and stability of transversely two-dimensional spatial solitons in a Kerr medium under conditions of small deviations from paraxial. Distributions of the transverse and longitudinal components of the soliton electric and magnetic fields are obtained. It is shown that the power of a nonparaxial soliton in a Kerr medium increases as the propagation constant increases. A linear analysis is made of soliton stability. In addition to confirming stability, this analysis revealed “internal modes” of nonparaxial solitons and their characteristics were determined. 相似文献
17.
分析了有限磁场中激光等离子体通道周围为有耗气体介质时激光引导电磁脉冲传播的一般模式的传播特性,建立了有限磁场中激光等离子体通道引导电磁脉冲的几何模型,导出了广义柱坐标系下各向异性介质中纵向场所满足的波动方程及纵向场与横向场的关系。利用边界条件给出了有限磁场中激光引导电磁脉冲传播模式的严格特征方程,重点讨论了传播常数随等离子体参数、周围介质参数和外加磁场的变化。结果表明,有限磁场中激光引导电磁脉冲的传播特性比无磁场或外加无穷大磁场时更具有可控性。 相似文献
18.
P. F. Smith 《Zeitschrift fur Physik C Particles and Fields》1986,31(2):265-275
Neutrinos from the sun or from a supernova will scatter coherently from target nuclei, and the associated nuclear recoil energy could in principle be detected using the significant local temperature rise produced at low temperatures in materials with a negligible electronic specific heat. Heavy ‘dark matter’ particles such as photinos might also be detected in this way. Drukier and Stodolsky [7] have suggested the use of a target in the form of superheated grains of superconductor which would be switched to the resistive state by individual neutrino scattering events, producing small but detectable local magnetic flux changes. The present paper considers the alternative scheme of a target consisting of coils of fine single or multi-filament superconducting wires, allowing the local resistive transitions to be detected as voltage pulses at the coil input. Calculations are presented of neutrino event rate versus energy deposited as a function of the target (A, Z) value, and the required filament diameter as a function of temperature and recoil energy, taking into account the latent heat requirements of the superconductor at transition. The possibility of using electrically parallel arrays of filaments is analysed, and the magnitude of the external voltage pulse is estimated for a range of type 1 and type 2 superconducting materials, including the effect of propagation of the normal zone. It is concluded that measurable voltage signals could in general be obtained with both type 1 and type 2 superconductors, and for operating temperatures in the region 10–100 mK typical filament diameters would range from 10–40 micron (for 30 eV recoil energy sensitivity) 40–160 micron (for 3 keV recoil energy sensitivity). 相似文献
19.
The magneto-gravitational instability of an infinite homogeneous, finitely conducting, viscous rotating plasma through porous medium is investigated in view of its relevance to certain stellar atmospheres. The dispersion relation has been obtained from the relevant linearized perturbation equations of the problem and it has been discussed in the case of rotation parallel and perpendicular to the direction of magnetic field separately. The longitudinal and transverse modes of wave propagation are discussed in each case of rotation. It is found that the combined effect of viscosity, finite conductivity, rotation and the medium porosity does not essentially change the Jeans' criterion of gravitational instability. It is also shown that for the propagation transverse to the direction of magnetic field. the finite conductivity destabilizes the wave number band which is stable in the limit of infinite conductivity when the medium is considered inviscid. 相似文献
20.